(1) Field of the invention
The present invention relates generally to a system and method for automatically controlling a vehicle speed to a desired cruise speed applicable to a vehicle.
(2) Background of the art
Recently, automatic vehicle speed controlling systems have been installed in vehicles by means of which the vehicles automatically run at desired cruising speeds (so called ASCD (Automatic Speed Control Devices).
In such vehicles, operators (drivers) accelerate the vehicles to reach speeds at which the drivers desire the vehicles to cruise. When the vehicles attain the desired speeds, the drivers recognize that the vehicles run at the desired cruising speeds through speedometers and the drivers set the vehicle speeds through set buttons. At this time, the systems adjust opening angles of throttle valves or carburetors so that the actual vehicle speeds coincide with the set vehicle speeds.
One such automatic vehicle speed controlling systems as described above is exemplified by A Japanese Patent Application First Publication (Tokkai) sho 59-58134 published on Apr. 3, 1984.
When such vehicle speed controlling systems are installed in the vehicles, it is convenient for the driver since no depression on an accelerator pedal is needed to cruise the vehicle during run on a freeway.
Furthermore, the automatic speed controlling function described above is released when the driver actuates a clutch pedal or brake pedal. In addition, when the accelerator pedal is depressed, the vehicle speed can exceed the set cruising speed. When the accelerator pedal depression is halted, the vehicle can again run at the set cruise speed after the speed reduction caused by an engine brake.
A microcomputer is installed which controls a throttle valve actuator according to a vehicle speed difference between the actual vehicle speed and set vehicle speed in the above-described vehicle speed controlling system. An opening angle of a throttle valve is adjusted through the actuation of the throttle actuator. Controlling means constituted by the microcomputer inhibits a power supply to the throttle actuator when the actual vehicle speed is a vehicle speed inappropriate for the cruising speed (for example, 45 Km/h or below) since it is uncontrollable speed.
However, the controlling means described above often erroneously detects the vehicle speed which is below the lowest controllable speed as the controllable vehicle speed so as to inappropriately control over the throttle actuator is resulted.